Milliwatt average power terahertz quantum cascade lasers (THz-QCLs) combined with microbolometer focal plane array cameras allow for acquisition rates on the order of 1×10<sup>6</sup> pixels/s. This system enables real-time imaging in transmission and reflection modes with signal to noise ratios of >25 dB per pixel. While these system allow rapid imaging for fairly transparent samples, signal to noise ratios of > 90 dB can be achieved with single element detectors where the samples are more opaque or require higher SNR. Systems using LongWave's terahertz QCLs and single/multi-element detectors will be presented.
Milliwatt average power terahertz quantum cascade lasers (THz-QCLs, 2 THz to 5 THz) have been developed for spectroscopy and as local oscillators for heterodyne receivers. Novel DFB THz-QCLs have been fabricated and show single-mode operation. The narrow line widths of <10 MHz and stark shift tuning of of 6 GHz, allows for wavelength modulation spectroscopy of low pressure gasses in the unexplored THz frequency band. The same devices also act as local-oscillators for heterodyne receivers for remote-sensing and astronomy. Lastly we report on improved tunable DFB devices for use in spectroscopy.
We report on the performance of a high sensitivity 4.7 THz heterodyne receiver based on a NbN hot electron bolometer mixer and a quantum cascade laser (QCL) as local oscillator. The receiver is developed to observe the astronomically important neutral atomic oxygen [OI] line at 4.7448 THz on a balloon based telescope. The single-line frequency control and improved beam pattern of QCL have taken advantage of a third-order distributed feedback structure. We measured a double sideband receiver noise temperature (T<sub>rec(DSB)</sub>) of 815 K, which is ~ 7 times the quantum noise limit (hν/2kB). An Allan time of 15 s at an effective noise fluctuation bandwidth of 18 MHz is demonstrated. Heterodyne performance was further supported by a measured methanol line spectrum around 4.7 THz.
By introducing coupled microstrip antennas on THz Distributed Feedback (DFB) Quantum Cascade Lasers (QCLs), the
radiation efficiency of each feedback aperture is greatly enhanced. Single mode emission ~3 THz from a 31-period
antenna-coupled third-order DFB laser yields ~4 times improvement in output power comparing with a corrugated thirdorder
device fabricated on the same gain medium. This 31-period device has ~15×25° beam divergence and 4 mW
pulsed power (4%) at 10 K with maximum lasing temperature (Tmax) at 134 K (pulsed). When phase matching condition
is met, emissions from 81 apertures (4-mm long) are coherently combined to form a narrow beam with 12.5° divergence.
Further experiment demonstrated the new device at 4 THz (25-period, ~18 μm×1-mm long. The 4 THz device reaches
>8 mW pulsed power (10%) at 12 K with Tmax 109 K (pulsed) and >77 K (cw). The slope efficiency is 450 mW/A with
0.57% wall-plug. It is worth pointing out although the antennas would be excited differently, similar enhancement in
out-coupling efficiency can also be observed in second-order surface-emitting THz DFB lasers. Begin the abstract two
lines below author names and addresses.
The interfaces of a dielectric sample are resolved in reflection geometry using light from a frequency agile array of
terahertz quantum-cascade lasers. The terahertz source is a 10-element linear array of third-order distributed feedback
QCLs emitting at discrete frequencies from 2.08 to 2.4 THz. Emission from the array is collimated and sent through a
Michelson interferometer, with the sample placed in one of the arms. Interference signals collected at each frequency are
used to reconstruct an interferogram and detect the interfaces in the sample. Due to the long coherence length of the
source, the interferometer arms need not be adjusted to the zero-path delay. A depth resolution of 360 μm in the
dielectric is achieved with further potential improvement through improved frequency coverage of the array. The entire
experiment footprint is <1 m x 1 m with the source operated in a compact, closed-cycle cryocooler.
We report a new experiment on a high-resolution heterodyne spectrometer using a 3.5 THz quantum cascade laser
(QCL) as local oscillator (LO) and a superconducting hot electron bolometer (HEB) as mixer by stabilizing both
frequency and amplitude of the QCL. The frequency locking of the QCL is demonstrated by using a methanol molecular
absorption line, a proportional-integral-derivative (PID) controller, and a direct power detector. We show that the LO
locked linewidth can be as narrow as 35 KHz. The LO power to the HEB is also stabilized by means of swing-arm
actuator placed in the beam path in combination of a second PID controller.
High-resolution heterodyne spectrometers operating at above 2 THz are crucial for detecting, e.g., the HD line at 2.7
THz and oxygen OI line at 4.7 THz in astronomy. The potential receiver technology is a combination of a hot electron
bolometer (HEB) mixer and a THz quantum cascade laser (QCL) local oscillator (LO).Here we report the first highresolution
heterodyne spectroscopy measurement of a gas cell using such a HEB-QCL receiver. The receiver employs a
2.9 THz free-running QCL as local oscillator and a NbN HEB as a mixer. By using methanol (CH<sub>3</sub>OH) gas as a signal
source, we successfully recorded the methanol emission line at 2.92195 THz. Spectral lines at IF frequency at different
pressures were measured using a FFTS and well fitted with a Lorentzian profile. Our gas cell measurement is a crucial
demonstration of the QCL as LO for practical heterodyne instruments. Together with our other experimental
demonstrations, such as using a QCL at 70 K to operate a HEB mixer and the phase locking of a QCL such a receiver is
in principle ready for a next step, which is to build a real instrument for any balloon-, air-, and space-borne observatory.